One application of an IPT system where individual control of the loads is desirable is inductively powered road-studs. These are now a recognized traffic control technique, and are described in U.S. Pat. No. 6,459,218. As described in U.S. Pat. No. 6,459,218, a very low frequency (VLF) current is passed through a conducting wire (which may be referred to as a primary conductor or track) buried under the roadway. Road-studs sensitive to the magnetic field created by that wire are placed on the surface of the roadway and take power from the buried conductor by mutually coupling some of the magnetic field through a pick-up coil in the road-stud. The pick-up coil may be tuned with a capacitor to augment the power transfer process. The power transferred may be controlled by any of a variety of methods and used to drive traffic control lights as required. For example, one control method is described in U.S. Pat. No. 5,293,308, the contents of which are incorporated herein by reference.
In a separate development, U.S. Pat. No. 6,317,338 describes an intermediate tuned circuit (which may be referred to as an active node) that may be placed between the buried wire and the pick-up coil. Although U.S. Pat. No. 6,317,338 refers to use of the intermediate circuit with electroluminescent displays, it has equal application with the apparatus of the present invention. The contents of U.S. Pat. No. 6,317,338 are incorporated herein by reference. The intermediate tuned circuit enhances the magnetic field so that more power may be transferred for a given current in the buried wire. In practice a buried wire requires a 15-20 A current at 38.4 kHz to drive typical road-studs whereas with active nodes the current may be reduced to typically 5 A. A current of 5 A allows the use of lower cost, thinner conductors, and generators can power longer lengths for a given voltage rating. These advantages are possible as the road-studs do not move so that the active nodes may be accurately located. Such an advantage is not possible with a moving IPT monorail system.
Road-studs powered in this way are environmentally inert and unaffected by roading or weather conditions. They are spark-free and safe, and the loss by removal of one stud or more has no effect on the performance of others in the string. They may be turned on or off by switching the current in the buried conductor on or off. In U.S. Pat. No. 6,459,218 reference is made to selectively addressing or controlling one or more lamp units in a series, to enable independent control of individual units. It is indicated that this may be achieved by varying the amplitude or frequency of the primary current. The present invention concerns specific apparatus and methods for achieving independent control of one or more lamp units or other loads in an IPT system.